This invention relates to supercritical fluid extraction and supercritical fluid chromatography and more particularly to the collection of extracted or separated sample in supercritical fluid extraction or supercritical fluid chromatography.
In supercritical fluid extraction, an extraction vessel is held at a temperature above the critical point and is supplied with fluid at a pressure above the critical pressure. Under these conditions, the fluid within the extraction vessel is a supercritical fluid. In supercritical fluid chromatography, a similar process is followed except that the supercritical fluid moves the sample through a column, separates some of the components of the sample one from the other and removes the components from the column.
In one class of supercritical fluid extraction of chemical components from a sample using a supercritical fluid, the components dissolved in the extraction fluid are separated from the fluid for further analysis by allowing the extraction fluid to vaporize.
In a prior art type of supercritical fluid extraction apparatus, the analyte precipitates on the surfaces of the expansion device, such as for example, along the walls of a linear capillary tube restrictor or on the walls of tubing beyond the limiting orifice of a point restrictor as the extraction fluid vaporizes. Commercially available metering valves as point restrictors require the analyte to be removed from the internal surface area of the connecting conduit or in tubing downstream from the valve in which it precipitates.
The analyte is collected in a collection solvent located in a collection vessel. In the prior art, one key advantage is achieved by collecting the analyte in collection solvent within the collection vessel which advantage is that volatile analytes are less likely to be lost by their own vaporization.
This loss occurs because, as the extraction fluid vaporizes, volatile analytes may also tend to vaporize and be lost with the extraction fluid. By collecting the effluent in supercritical fluid extraction and in supercritical fluid chromatography in a collection solvent, volatile compounds tend to be dissolved in the collection solvent rather than being lost with the expanded supercritical fluid, which is a gas after expansion. The higher recovery rate of volatile analytes is advantageous when the content of volatile compounds in the sample is small and when the volatile content is to be quantified.
The prior art apparatuses and methods for collecting sample have the disadvantages of requiring an excessive amount of time and equipment to remove analytes from tubing and of losing some analytes.
In collecting sample (analytes) during supercritical fluid extraction and supercritical fluid chromatography, a fluid flow restrictor is included to maintain high pressure in an extraction chamber or column while allowing a controlled flow rate through the sample being extracted. One type of restrictor is a length of small internal diameter tubing, often referred to as a capillary restrictor or capillary.
To avoid freezing or deposition of water or other extracted substances dissolved in the fluid on the wall of the tubing, the capillary is heated. The need for heating is especially great when using a cold collection trap comprising a cold collection liquid solvent in which the outlet end of the capillary is immersed and through which gasified extractant is bubbled.
In one prior art heated restrictor, the capillary is heated by thermal conduction along its length and by heat or enthalpy added to the fluid within the capillary, which moves along with the fluid flow to the outlet end of the capillary. The fluid discharges into a cold, dry tube of relatively large inside diameter. This larger tube then dips into the cold solvent trap. Ice and extracts build up on this tube but do not plug it because of the large diameter. This is described in international patent application number WO 92/06058, dated Apr. 14, 1992.
This arrangement is disadvantageous because it is often difficult to remove extract solidified on the inside of the large tube for assay.
It is known to directly resistance heat a member and to control the heat with a feedback system using the electrical resistance of the member to measure its temperature and compare it to a reference temperature. This technique is taught for use in a gas tube by U.S. Pat. No. 4,438,370; the disclosure of which is incorporated herein by reference.
In another restrictor-collector system that may or may not be prior art, a heated variable restrictor is mounted within a heating block. A tube extends from the heated variable restrictor into the collection trap. This type of variable restrictor may still have the disadvantage of depositing extract on the tubular walls of the tube that extends from the heated variable restrictor into the collection trap. A system of this type is described by Maxwell, et al. in "improved SFE Recovery of Trace Analytes from Liver Using an Integral Micrometering Valve-SPE Column Holder Assembly" Journal of High Resolution Chromatorgraphy v. Dec. 15, 1992, pp. 807-811.